Fuel economy of vehicles has been conventionally improved by reducing rolling resistance of tires to suppress the heat build-up. In recent years, with the increase in demand for fuel-efficient vehicles, not only the tread occupying a larger portion of a tire but also other tire components such as sidewall have been needed to contribute to an improvement in fuel efficiency. Moreover, fuel-efficient tires for passenger cars as well as for trucks and buses are needed. Therefore, it is also important to improve abrasion resistance at the same time.
The performance in terms of low heat build-up can be improved by, for example, using a semi-reinforcing filler in a rubber composition or reducing the amount of filler in a rubber composition, but on the other hand abrasion resistance and tensile properties are then reduced. Therefore, such properties are difficult to achieve at the same time.
Moreover, silica is used for the purpose of reducing rolling resistance, but silica strongly self-aggregates and is therefore difficult to uniformly disperse by an ordinary kneading method. In particular, finely divided silica is more likely to strongly aggregate. In addition, a chemical reaction of silica with a silane coupling agent needs large amounts of energy and time. Therefore, such a reaction is difficult to sufficiently progress by an ordinary kneading method. Further, the unreacted silane coupling agent generates alcohol (e.g. ethanol), which disadvantageously creates bubbles in a formed article.
Meanwhile, natural rubber has poor processability compared to other synthetic rubbers, and is usually used after being subjected to mastication. Therefore, natural rubber generally has poor productivity. Further, the mastication causes molecular chain scission in natural rubber, which poses the problem that natural rubber loses its original properties as a high-molecular-weight polymer (e.g., fuel efficiency, abrasion resistance, tensile properties).
Patent Literature 1 discloses a composite prepared by mixing a natural rubber latex with finely divided silica formed from a water glass, as a rubber masterbatch having improved mechanical strength. However, sufficient reaction of a silane coupling agent is not sufficiently examined in this document. Therefore, there is room for improvement in achieving fuel efficiency, abrasion resistance, and tensile properties at the same time.